Mutagenesis is induced by physical and chemical means provoking DNA damages when incorrectly repaired leading to mutations. Several chemicals are known to cause DNA lesions and are routinely used. Radiomimetic agents work through free radical attack on the sugar moieties of DNA (Povirk 1996). A second group of drugs inducing DNA damage includes inhibitors of topoisomerase I (TopoI) and II (TopoII) (Teicher 2008) (Burden and N. 1998). Other classes of chemicals bind covalently to the DNA and form bulky adducts that are repaired by the nucleotide excision repair (NER) system (Nouspikel 2009). Chemicals inducing DNA damage have a diverse range of applications and are widely used. However, although certain agents are more commonly applied in studying a particular repair pathway (e.g. cross-linking agents are favored for NER studies), most drugs simultaneously provoke a variety of lesions (Nagy and Soutoglou 2009). The physical means to generate mutagenesis is through the exposure of cells to ionizing radiation of one of three classes—X-rays, gamma rays, or neutrons (Green and Roderick 1966). However, using these classical, strategies, the overall yield of induced mutations is quite low, and the DNA damage leading to mutagenesis cannot be targeted to precise genomic DNA sequence.
The most widely used in vivo site-directed mutagenesis strategy is gene targeting (GT) via homologous recombination (HR). Efficient GT procedures have been available for more than 20 years in yeast (Rothstein 1991) and mouse (Capecchi 1989). Successful GT has also been achieved in Arabidopsis and rice plants (Hanin, Volrath et al. 2001; Terada, Urawa et al. 2002; Endo, Osakabe et al. 2006; Endo, Osakabe et al. 2007). Typically, GT events occur in a fairly small proportion of treated mammalian while GT efficiency is extremely low in higher plant cells and range between 0.01-0.1% of the total number of random integration events (Terada, Johzuka-Hisatomi et al. 2007). The low GT frequencies reported in various organisms are thought to result from competition between HR and non homologous end joining (NHEJ) for repair of dsDNA breaks (DSBs). As a consequence, the ends of a donor molecule are likely to be joined by NHEJ rather than participating in HR, thus reducing GT frequency. There is extensive data indicating that DSBs repair by NHEJ is error-prone. Often, DSBs are repaired by end-joining processes that generate insertions and/or deletions (Britt 1999). Thus, these NHEJ-based strategies might be more effective than HR-based strategies for targeted mutagenesis into cells. Indeed, expression of I-Sce I, a rare cutting restriction enzyme, has been shown to introduce mutations at I-Sce I cleavage sites in Arabidopsis and tobacco (Kirik, Salomon et al. 2000). Nevertheless, the use of restriction enzymes is limited to rarely occurring natural recognition sites or to artificial target sites. To overcome this problem, meganucleases with engineered specificity towards a chosen sequence have been developed. Meganucleases show high specificity to their DNA target, these proteins being able to cleave a unique chromosomal sequence and therefore do not affect global genome integrity. Natural meganucleases are essentially represented by homing endonucleases, a widespread class of proteins found in eukaryotes, bacteria and archae (Chevalier and Stoddard 2001). Early studies of the I-Sce I and HO homing endonucleases have illustrated how the cleavage activity of these proteins can be used to initiate HR events in living cells and have demonstrated the recombinogenie properties of chromosomal DSBs (Dujon, Colleaux et al. 1986; Haber 1995). Since then, meganuclease-induced HR has been successfully used for genome engineering purposes in bacteria (Posfai, Kolisnychenko et al. 1.999), mammalian cells (Sargent, Brenneman et al. 1997; Cohen-Tannoudji, Robine et al. 1998; Donoho, Jasin et al. 1998), mice (Cbuble, Smith et al. 2006) and plants (Puchta, Dujon et al. 1996; Siebert and Puchta 2002). Meganucleases have emerged as scaffolds of choice for deriving genome engineering tools cutting a desired target sequence (Paques and Duchateau 2007).
Combinatorial assembly processes allowing to engineer meganucleases with modified specificities has been described by Arnould et al. (Arnould, Chames et al. 2006; Smith, Grizot et al. 2006; Arnould, Perez et al. 2007; Grizot, Smith et al. 2009). Briefly, these processes rely on the identifications of locally engineered variants with a substrate specificity that differs from the substrate specificity of the wild-type meganuclease by only a few nucleotides. An other type of specific endonucleases is based on Zinc finger nuclease. ZFNs are chimeric proteins composed of a synthetic zinc finger-based DNA binding domain and a DNA cleavage domain. By modification of the zinc finger DNA binding domain, ZFNs can be specifically designed to cleave virtually any long stretch of dsDNA sequence (Kim, Cha et al. 1996; Cathomen and Joung 2008). An NHEJ-based targeted mutagenesis strategy was developed recently in several organisms by using synthetic ZFNs to generate DSBs at specific genomic sites (Lloyd, Plaisier et al. 2005; Beumer, Trautman et al. 2008; Doyon, McCammon et al. 2008; Meng, Noyes et al. 2008). Subsequent repair of the DSBs by NHEJ frequently produces deletions and/or insertions at the joining site. For examples, in zebrafish embryos, the injection of mRNA coding for engineered ZFN led to animals carrying the desired heritable mutations (Doyon, McCammon et al. 2008). In plant, same NHEJ-based targeted-mutagenesis has also been successful (Lloyd, Plaisier et al. 2005). Although these powerful tools are available, there is still a need to further improved double-strand break-induced mutagenesis.
As mentioned above, two mechanisms for the repair of DSBs have been described, involving either homologous recombination or non-homologous end-joining (NHEJ). NHEJ consists of at least two genetically and biochemically distinct process (Feldmann, Schmiemann et al. 2000). The major and best characterized “classic” end-joining pathway (C-NHEJ) involves rejoining of what remains of the two DNA ends through direct, relegation (Critchlow and Jackson 1998). A scheme for this pathway is shown in FIG. 1. NHEJ can be divided in three major steps: detection and protection of DNA ends, DNA end-processing and finally DNA ligation, Detection and protection of DNA ends are mediated by DNA-PK which is composed of Ku70 and Ku80 proteins that form an heterodimer (Ku) binding DNA ends and recruiting DNA-PK catalytic subunit (DNA-PKcs). This interaction DNA-PKcs-Ku-DSB stimulates DNA-PKcs kinase activity, maintains the broken ends in close proximity and prevents from extended degradation. Ku also recruits other components of C-NHEJ repair process. Candidates for DNA end processing are Artemis DNA polymerase mu (μ) and lamda (λ), polynucleotide kinase (PNK) and Werner's syndrome helicase (WRN) (for review (Mahaney, Meek et al, 2009)). The ligation process is mediated by DNA ligase IV and its cofactors XRCC4 and XLF/Cernnunos. Finally, other proteins or complex modulating NHEJ activity have been described such as BRCA1, Rad50-Mre11-Nbs (Williams, Williams et al. 2007; Shrivastav, De Haro et al. 2008) complex, CtIP or FANCD2 (Bau, Man et al. 2006; Pace, Mosedale et al. 2010)). NHEJ is thought to be effective at all times in the cell cycle ((Essers, van Steeg et al. 2000); (Takata, Sasaki et al. 1998)). NHEJ also plays an important role in DSB repair during V(D)J recombination (Blunt, Finnie et al. 1995) (Taccioli, Rathbun et al. 1993).
The second mechanism, referred as microhomology mediated end joining (MMEJ) or alternative NHEJ (A-NHEJ) or back up NHEJ (B-NHEJ) is associated with significant 5′-3′ resection of the end and uses microhomologies to anneal DNA allowing repair. Little is known about the components of this machinery. DNA ligase3 with XRCC1 proteins are candidate for the ligase activity (Audebert, Salles et al. 2004; Wang, Rosidi et al. 2005). PARP seems also to be an important factor of this mechanism (Audebert, Salles et al. 2004) (Wang, Wu et al. 2006).
Theoretically, both classical and alternative NHEJ could lead to mutagenesis, although A-NHEJ mechanism would represent the main pathway to favour when one wants to increase DSB-induced mutagenesis. Several methods have been described in order to modulate NHEJ. For example, US 2004/029130 A1 concerns a method of stimulating NHEJ of DNA the method comprising performing NHEJ of DNA in the presence of inositol hexakisphosphate (IP6) or other stimulatory inositol phosphate. The invention also provides screening assays for compounds which may modulate NHEJ and DNA-PK and related protein kinases and which may be therapeutically useful. WO 98/30902 relates to modulation of the NHEJ system via regulation (using protein and/or natural or synthetic compounds) of the interactions of XRCC4 and DNA ligase IV, and XRCC4 and DNA-PK to effect cellular DNA repair activity. It also relates to screens for individuals predisposed to conditions in which XRCC4 and/or DNA ligase IV are deficient, Sarkaria et al. (Sarkaria, Tibbetts et al. 1998) describes the inhibition of phosphoinositide 3-kinase related kinases (such as DNA-dependent protein kinase, ATR and ATM) by the radiosensitizing agent, wortmannin.
In an attempt to define in molecular detail the mechanism of NHEJ, an in vitro system for end-joining was recently developed (Baumarm and West 1998). The reactions exhibited an apparent requirement for DNA-PKS, Ku70/80, XRCC4 and DNA ligase IV, consistent with the in vivo requirements. Preliminary fractionation and complementation assays, however, revealed that these factors were not sufficient for efficient end-joining, and that other components of the reaction remained to be identified.
RNA interference is an endogenous gene silencing pathway that responds to dsRNAs by silencing homologous genes (Meister and Tuschl 2004). First described in Caenorhabditis elegans by Fire et al. the RNAi pathway functions in a broad range of eukaryotic organisms (Hannon 2002). Silencing in these initial experiments was triggered by introduction of long dsRNA. The enzyme Dicer cleaves these long dsRNAs into short-interfering RNAs (siRNAs) of approximately 21-23 nucleotides. One of the two siRNA strands is then incorporated into an RNA-induced silencing complex (RISC). RISC compares these “guide RNAs” to RNAs in the cell and efficiently cleaves target RNAs containing sequences that are perfectly, or nearly perfectly complementary to the guide RNA.
For many years it was unclear whether the RNAi pathway was functional in cultured mammalian cells and in whole mammals. However, Elbashir S. M. et al, 2001 (Elbashir, Harborth et al. 2001), triggered RNAi in cultured mammalian cells by transfecting them with 21 nucleotide synthetic RNA duplexes that mimicked endogenous siRNAs. McCaffrey et al. (McCaffrey, Meuse et al, 2002), also demonstrated that siRNAs and shRNAs could efficiently silence genes in adult mice.
Introduction of chemically synthetized siRNAs can effectively mediate post-transcriptional gene silencing in mammalian cells without inducing interferon responses. Synthetic siRNAs, targeted against a variety of genes have been successfully used in mammalian cells to prevent expression of target mRNA (Harborth, Elbashir et al. 2001). These discoveries of RNAi and siRNA-mediated gene silencing has led to a spectrum of opportunities for functional genomics, target validation, and the development of siRNA-based therapeutics, making it a potentially powerful tool for therapeutics and in vivo studies.
The authors of the present invention have developed a new approach to increase the efficiency of DSB-induced mutagenesis. This new approach relates through, the identification of new effectors that modulate said DSB-induced mutagenesis by uses of interfering agents in an in vivo assay. These agents being capable of modulate DSB-induced mutagenesis through their respective direct or indirect actions on respective effectors, introduction of these interfering agents and/or derivatives into a cell, respectively, will lead to a cell wherein said DSB-induced mutagenesis is modulated.